The present invention relates generally to methods for producing silicon and more particularly to a method for producing solar-cell-grade silicon which may be utilized as a material of construction for photovoltaic cells which convert solar energy directly into electrical energy.
Silicon employed in photovoltaic cells must have a relatively high degree of purity in order efficiently to convert solar energy into electrical energy. It is particularly important for the solar-cell-grade silicon to have a very low amount of elements from groups III and V of the Periodic Table of Elements, especially boron and phosphorous.
Silicon employed as a material for semiconductors has a relatively high degree of purity, but semiconductor-grade silicon is produced by a relatively expensive process, and the expense is too great to be commercially practical for solar-cell-grade silicon.
It would be desirable to produce solar-cell-grade silicon employing a simple, relatively inexpensive smelting process heretofore utilized to produce metallurgical-grade silicon. In this smelting process, silica (SiO.sub.2) is reduced by a carbonaceous reducing agent in a high temperature furnace, such as a direct arc reactor (DAR), or any type of electric furnace or an arc plasma furnace. However, the silicon produced by such a method has a relatively high degree of impurities (e.g. only about 98% pure). These impurities comprise metals, such as Al, Cr, Fe, Ti, V and Zr, among others, as well as boron and phosphorous. Most of the metallic impurities in metallurgical-grade silicon can be removed with purifying processes after the smelting operation. However, there is no known process for reducing boron and phosphorous under commercially practicable conditions to levels sufficiently low as to no longer significantly impair the efficiency of the solar cell to convert solar energy directly to electrical energy. Although boron and phosphorous are reduced to these low levels in the production of semiconductor-grade silicon, the process for doing so is too expensive to be commercially practicable for solar-cell-grade silicon.
In the production of metallurgical-grade silicon, a typical carbonaceous reducing agent is composed of a mixture of wood chips, coal and coke. This particular carbonaceous reducing agent is highly reactive with the silica and produces 75-85% yields of silicon, but the resulting silicon has relatively large amounts of boron and phosphorous, too large to produce high-efficiency solar cells. Unpurified charcoal, another carbonaceous reducing agent utilized in a method employing a direct arc reactor, also has a relatively high degree of reactivity with the silica and produces relatively high yeilds of silicon, but, like the carbonaceous reducing agents comprising a mixture of wood chips, coal and coke, the unpurified charcoal also produces silicon having relatively large amounts of boron and phosphorous.
Another possible carbonaceous reducing agent for silica is petroleum coke, but this carbonaceous agent, by itself, is a rather poor reductant for silica, and the yields of silicon employing this particular carbonaceous reducing agent are relatively low.
In summary, the carbonaceous reducing agents heretofore employed in the reduction of silica in a direct arc reactor either have produced silicon with excessive boron and phosporous contents, or they have a relatively low reactivity with silica, giving a rather low yield of silicon and thus are not practical from a commercial standpoint.